1. Field of the Invention
The present invention relates to a method for forming a compound semiconductor thin film containing a ZnO-based group II-VI compound semiconductor as a principal constituent, and to a light emitting device using the same.
2. Description of the Related Art
ZnO, which is a group II-VI compound semiconductor, is receiving attention as a material for light emitting devices for emitting light in the ultraviolet band, and since it is of a direct gap type, the light emission efficiency is high.
In a light emitting device, such as a light emitting diode or a semiconductor laser, a pn junction comprising a p-type semiconductor layer and an n-type semiconductor layer is formed on a substrate. Accordingly, in order to fabricate a light emitting device, a p-type semiconductor layer and an n-type semiconductor layer must be grown on a substrate. When a semiconductor layer of one conductive type is epitaxially grown on a substrate and a semiconductor layer of another conductive type is epitaxially grown thereon, it is preferable that lattice mismatching does not occur and crystal growth be carried out in a continuous process. Therefore, when a light emitting device is fabricated, it is preferable that semiconductor layers of different conductive types be deposited using the same semiconductor material.
However, in the conventional ZnO-based group II-VI compound semiconductor, it is only possible to form a single crystal or thin film of n-type conductivity, and moreover, it is only possible to form a single crystal or thin film with low resistivity.
Therefore, when a ZnO single crystal or thin film having high resistivity is required, the resistance is increased by doping the ZnO single crystal or thin film with a compensating acceptor, such as Li, Cu, or Ag. In such a case, however, only the n-type is produced, and even with the doping of impurities, it is not possible to obtain a p-type ZnO thin film. Consequently, in order to obtain a pn junction using the group II-VI compound semiconductor, the group II-VI compound semiconductor must be combined with a semiconductor of a different type, and thus lattice mismatching often occurs.
Accordingly, it is an object of the present invention to provide a method for forming a novel p-type semiconductor film including a semiconductor layer containing a compound semiconductor as a principal constituent doped with an impurity.
The above and other objects of the invention are accomplished by a method for forming a p-type semiconductor film comprising the steps of providing on a substrate a group II-VI compound semiconductor film which is doped with a p-type impurity and comprises ZnO; and activating the p-type impurity by annealing the doped semiconductor film.
In one aspect of the present invention, a method for forming a p-type semiconductor film comprises the steps of providing on a substrate a group II-VI compound semiconductor film which is doped with a p-type impurity and comprises either MgXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) or CdXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) and activating the p-type impurity by annealing the doped semiconductor film.
The semiconductor film providing step may include the step of sputtering a target doped with a p-type impurity and made of either MgXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) or CdXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) to form the semiconductor film on the substrate. Alternatively, the semiconductor film providing step may include the steps of sputtering a target made of either MgXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) or CdXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) to form the semiconductor film on the substrate and doping the semiconductor film with the p-type impurity. In this case, the semiconductor film may be doped with the p-type impurity by an ion implanting method.
Preferably, the p-type impurity includes an element selected from the group consisting of As, N, P, Sb, and Bi. Preferably, the semiconductor film is annealed at 450xc2x0 C. or more.
By experimentation, it has been found that, by doping a semiconductor layer containing a group II-VI compound semiconductor selected from the group consisting of MgXZn1xe2x88x92XO and CdXZn1xe2x88x92XO as a principal constituent with any one of As, N, P, Sb, and Bi, followed by annealing, preferably at a temperature of 450xc2x0 C. or more for at least one hour, a p-type semiconductor layer comprising the group II-VI compound semiconductor can be obtained.
Preferably, the semiconductor layer has a resistivity of 106 xcexa9xc2x7cm or more before being doped with the impurity to obtain the p-type semiconductor layer. In the semiconductor layer having such a high resistance, the ZnO-based film has a small amount of oxygen defects. In the film having such a large resistivity, the resistance is easily decreased by doping. Preferably, the ZnO-based film is formed, in an atmosphere containing a sufficient amount of oxygen, using the ZnO-based material having a purity of 99.999% (5N) or more, and more preferably 99.9995% or more.
In another aspect of the present invention, a light emitting device includes a group II-VI compound semiconductor film containing p-type MgXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) or CdXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) formed by the method described above, and a group II-VI compound semiconductor film containing n-type MgXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61) or CdXZn1xe2x88x92XO (0xe2x89xa6Xxe2x89xa61).
In accordance with the present invention, it is possible to obtain a group II-VI compound semiconductor thin film with p-type conductivity. When a light emitting device is fabricated, it is possible to combine a p-type group II-VI compound semiconductor thin film and an n-type group II-VI compound semiconductor thin film. In particular, by using ZnO as the group II-VI compound semiconductor, it is possible to fabricate a light emitting device having an emission wavelength spectrum in the ultraviolet band.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.